The invention relates to a sealing element for parts mounted so that they can rotate counter to each other, wherein at least two corresponding sealing surfaces have a tractrix shape. In addition, the invention relates to seals for roller bearings and pressure-resistant sleeves for the inflow or outflow of fluids in shaft bores.
Seals for rotatably mounted parts are known with a plurality of different work principles and geometric embodiments. For example, gland seals are based on the fact that a soft material opposite the rotating part is pressed against the rotating part by a corresponding device with a defined force. Here, due to the contact of the rotating part on the stationary gland seal, friction is produced, which leads to a torque, for example, on the shaft, wear on the gland seal and the shaft, and thus also to abraded parts.
For so-called self-sealing packing rings, in particular radial sealing rings, the sealing effect against a rotating shaft is also based on one or more sealing lips, which are in contact with the rotating shaft. Here, a spring integrated in the sealing lip is often provided for the necessary dimensional accuracy and contact force. Accordingly, in the design of the seal, a compromise must be accepted between the contact force of the sealing element and the size of the resulting, friction-dependent braking moment, as well as the generated wear. Not least of all, for the given sealing effect, a compromise must be accepted between a harder sealing material with longer service life, but usually higher required contact pressure and greater wear on the shaft, as well as a softer sealing material with usually lower contact pressure but often shorter service life.
For better understanding, it is noted both here and below, that the arrangements do not depend on the allocation of the seal to a rotating part, like a shaft, or a stationary part, like a housing or an axle. The decisive factor is merely the relative rotational movement of a part with a sealing function relative to another part.
Here, a rotating part is understood to refer not explicitly to only a pivotable part that in normal operation is rotated by only a fraction of a complete revolution. Also, e.g., the spindle of a valve, which can rotate between its end positions typically by a few up to a maximum of a few dozen revolutions, is applicable in this sense not as a rotating part but instead as a merely pivoting part. The loading in this case and the stated requirements are different and for the most part considerably smaller than in the case of a seal for rotating parts.
A rotating part distinguishes itself in the sense of this publication in that, in principle, it can execute a nearly unlimited number of revolutions relative to a stationary part and also executes a considerable number of revolutions relative to the stationary part for typical, function-specific use.
Thus, the servo axle of a modular-construction servo is only a pivoting part, while the shaft of a stepper motor of a printer-head drive is to be considered as a rotating part due to the plurality of revolutions from one end of the printing path to the other and the continuous motion during the operation of the printer. Roller-bearing seals are to be considered as rotating parts in principle and independent of their actual installation location and purpose of use, because according to their typical, function-specific use, they are designed for a very high number of revolutions.
For simplification, it is further defined that a sealing element or a sealing surface is a pivotable part or surface, which can be locked in rotation, for example, on a shaft, while a sealing counter element or a sealing counter surface is for the most part an essentially stationary element or an essentially stationary surface, which interacts with the sealing element, so that a sealing effect is achieved even for a relative rotation between these parts.
To reduce wear of the sealing surface and the sealing counter surface, the material of the seal is adapted to the material of the sealing counter surface and the appropriate conditions of use. It is further known to construct the shape, for example, of a sealing element, so that, e.g., a tight and reliable seat is guaranteed on a shaft and the seal still works satisfactorily for a long time even with the appearance of wear.
However, according to the prior state of the art, only characteristics of the sealing surface or sealing counter surface that offer advantages in terms of certain conditions of use relative to other seals are known. The common characteristic for them is that their sealing effect for the most part decreases rapidly for advancing wear of the sealing surfaces and/or the sealing counter surfaces.
It is further known that the tractrix curve, also called a hauling curve, drawing curve, pulling curve, or towing curve, is a transcendent curve, which is produced, for example, when a load, e.g., a watch, is pulled on a chain over a plane. In the case of primary interest here, the “actual” or straight tractrix curve, also called Huygens Tractrix after Christian Huygens, who first solved the underlying problem in 1693, the end of the watch away from the watch is pulled parallel along a straight line, e.g., the table edge. Here, the watch chain, in general the distance between the contact point and the coordinate axis, is constant in terms of its length.
If A0 is the starting point of the “drawing part” and P0 is the starting point of the “drawn part” and d is the distance A0P0>0 (corresponds to the length of the watch chain), and the point A of the drawing part travels on a straight line at the table edge, and also the point P “follows” the drawn watch at a constant distance A, then P traverses a Huygens tractrix.
The function equation of the Huygens tractrix reads in Cartesian coordinates:
      X    =                  A        ⁡                  [                                    cosh                              -                1                                      ⁡                          (                              A                /                Y                            )                                ]                    -                        (                                    A              2                        -                          Y              2                                )                      or            y      ⁡              (        x        )              =                                        ±            d                    ·          ar                ⁢                                  ⁢        cosh        ⁢                                  ⁢                  d          x                    ±                        (                                    d              2                        -                          x              2                                )                    
Because arcosh z can be expanded by ln z, this equation can also be written as
      y    ⁡          (      x      )        =                              ±          d                ·        ln            ⁢                                            d            +                                          (                                                      d                    2                                    -                                      x                    2                                                  )                                              x                              ±                  (                              d            2                    -                      x            2                          )            
In general, that is, in particular for cases, in which the guide curve (the path of the point A, in the previous example along a straight table edge) is not a straight line coinciding with a coordinate axis, but instead is any curve lying in the plane, the tractrix or hauling curve can be expressed as follows:
Let there be given a parameter t, a (guide) curve k, an arbitrary (starting) point A0, which lies on the curve k, and an arbitrary point P0. Here, let D be the distance between the points A0 and P0. Now if point A(t) traverses along the curve k with A(0)=A0 with increasing t, then it “follows” the point P(t) with P(0)=P0 at a constant distance d. The set of all points that P(t) traverses is designated as the tractrix of curve k:
      A    ⁡          (      t      )        =            P      ⁡              (        t        )              +          d      ·                                    P            .                    ⁡                      (            t            )                                                                        P              .                        ⁡                          (              t              )                                                    with {dot over (P)}(t)≠0.
Now if a tractrix, in particular a Huygens tractrix, is rotated about its Y-axis, then one obtains a pseudosphere with an equitangential boundary line, which is called a tractrix surface below. Rotation of only one section of the Huygens tractrix delivers a corresponding body, which describes a frustum with curved outer line and which is designated as a tractrix body below.
A known property of the tractrix surface or the tractrix body is that the resulting loading is distributed uniformly as a force over the surface for an axial pressure. The forces always act perpendicular to the surface. Plastic material creep due to forces acting diagonal to the surface is thus ruled out.
A practical application of the tractrix surface or the tractrix body has become known from the shape of a plug for a tap. For example, U.S. Pat. Nos. U.S. Pat. No. 4,878,652 B1 and U.S. Pat. No. 6,484,999 B1 each disclose plugs in the form of tractrix bodies, which exhibit an especially uniform pressure loading of the plug and thus a uniform, as well as, therefore especially low wear and low operating forces. Publications U.S. Pat. No. 5,044,606 A and U.S. Pat. No. 6,651,957 B2 disclose similar taps, wherein, however, the operating spindles of the plug also have in their sealing area tractrix bodies, which provide, together with a correspondingly shaped counter body in the form of a tractrix sleeve, not only for low and uniform wear, but also such that, for advancing wear of the surfaces, the tractrix form is always reproduced and the good sealing and running properties of the spindle seal are thus maintained. Here there is no suggestion for using tractrix bodies for seals, which are moved only occasionally and in the normal case only over a maximum of a quarter revolution.
From DE 199 41 361 A1, a sealing unit is known, which has two sealing rings that are arranged at a distance from each other and within annular grooves of a holder, in order to form a seal between a cylindrical shaft and a housing bore. Here, in the area of the sealing rings the holder contacts either the shaft or the housing and is thus locked in rotation with the sealing rings. The holder is created, so that it contacts neither the shaft nor the housing in an area between the sealing rings and also has openings, through which a lubricant is led from a bore in the housing into the area of the shaft between the sealing rings and can be led from there through a radial bore of the shaft into an axial bore of the same. The sealing rings can have different shapes, such as O-rings, continuous rings, lip seals, and labyrinth seals.